Hybrid electric vehicles low in CO2 emissions and electric vehicles free of CO2 emissions (hereinafter generically referred to as “electricity-powered vehicles”) are mounted with power supply units as means for driving traction motors. This type of power supply unit includes as many cell modules, in each of which a plurality of battery cells are electrically connected and packaged into a single unit, as are consistent in number with the required specifications of the electricity-powered vehicle concerned. A high-power secondary battery, such as a lithium-ion battery cell or a nickel-hydrogen battery cell, is used for each battery cell.
These batteries are susceptible to overcharge and overdischarge, however. Unless used in a prescribed range of charging voltages, these batteries may have such problems that materials decompose to significantly degrade capacity or the batteries generate unusual heat, thus becoming no longer unusable. In order to prevent these problems, the upper and lower voltage limits of a battery needs to be explicitly defined to control the charge and discharge of the battery, so that the terminal voltages of the battery fall within the upper and lower voltage limits.
Hence, as disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2010-282816, paragraphs 0017 and 0018), this type of power supply unit is provided with cell monitor unit (CMU), one each in respective cell modules or one each in a group of several cell modules, used to independently monitor each battery cell within the above-mentioned range, specifically cell monitor units used to monitor at least one of the voltage, current and temperature of each battery cell within the abovementioned range. In addition, the power supply unit is provided with a battery management unit (BMU) for performing integrated monitoring or integrated managing of respective cell monitor units.
According to Patent Document 1, one end of an electroconductive and thermally-conductive metal plate 18 is connected to an electrode terminal 12 of a battery cell 11, and a metal terminal 20 is joined to the other end of the metal plate 18. The metal terminal 20 is electrically and thermally joined to the electrode terminal 12 through the metal plate 18, so as to be able to detect the voltage of the battery cell 11. A battery monitoring apparatus board 31, which is a CMU 30, is disposed on the upper side of the metal plate 18 across a plurality of battery cells 11 in a cell module 10 and is fastened at the metal terminal 20.
Incidentally, if, for example, a cell monitor unit becomes damaged due to aging or for some other reason, though extremely unlikely, the unit has to be replaced with new one. In the cell module disclosed in the aforesaid Patent Document 1, however, the cell monitor unit cannot be easily detached because of the structure of the cell module. Accordingly, the cell module has the problem that the work of replacing the cell monitor unit is cumbersome and complicated. This also holds true when the cell monitor unit is temporarily detached for reasons of, for example, verifying the operation of the cell monitor unit.
Also in the cell module disclosed in Patent Document 1, the cell monitor unit is exposed. Accordingly, the cell module has the problem that the cell monitor unit is susceptible to static electricity. Such a problem can occur not only in a period in which the cell monitor unit is attached to the cell module but also at the time of replacing the cell monitor unit with new one as described above.
These types of problems are not limited to battery cells and cell monitor units, but are also true for capacitors (electric double layer capacitors and the like) and other circuits.